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                    Introduction
            
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                    Architecture Overview
            
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                    Axon Framework Extensions
            
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                    Introduction
            
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                    Messaging Concepts
            
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                    Anatomy of a Message
            
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                    Supported Parameters for Annotated Handlers
            
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                    Exception Handling
            
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                    Unit of Work
            
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                    Commands
            
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                    Modeling
            
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                    Aggregate
            
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                    Multi-Entity Aggregates
            
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                    State Stored Aggregates
            
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                    Aggregate Creation from another Aggregate
            
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                    Aggregate Polymorphism
            
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                    Conflict Resolution
            
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                    Command Dispatchers
            
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                    Command Handlers
            
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                    Infrastructure
            
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                    Configuration
            
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                    Events
            
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                    Event Dispatchers
            
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                    Event Handlers
            
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                    Event Processors
            
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                    Subscribing Event Processors
            
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                    Streaming Event Processors
            
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                    Event Bus & Event Store
            
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                    Event Versioning
            
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                    Event Serialization
            
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                    Queries
            
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                    Query Processing
            
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                    Query Dispatchers
            
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                    Query Handlers
            
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                    Implementations
            
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                    Configuration
            
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                    Sagas
            
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                    Implementation
            
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                    Associations
            
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                    Infrastructure
            
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                    Deadlines
            
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                    Deadline Managers
            
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                    Event Schedulers
            
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        <li class="chapter " data-level="3.8" data-path="../testing/">
            
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                    Testing
            
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                    Commands / Events
            
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        <li class="chapter " data-level="3.8.2" data-path="../testing/sagas-1.html">
            
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                    Sagas
            
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        <li class="chapter " data-level="3.9" data-path="../tuning/">
            
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                    Tuning
            
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                    Event Snapshots
            
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                    Event Processing
            
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        <li class="chapter " data-level="3.9.3" data-path="../tuning/command-processing.html">
            
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                    Command Processing
            
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                    Monitoring and Metrics
            
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        <li class="chapter " data-level="3.11" data-path="../spring-boot-integration.html">
            
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                    Spring Boot Integration
            
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                    Modules
            
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                    Introduction
            
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        <li class="chapter " data-level="4.2" data-path="../../axon-server/installation/">
            
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                    Installation
            
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        <li class="chapter " data-level="4.2.1" data-path="../../axon-server/installation/local-installation/">
            
                <a href="../../axon-server/installation/local-installation/">
            
                    
                    Local Installation
            
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        <li class="chapter " data-level="4.2.1.1" data-path="../../axon-server/installation/local-installation/axon-server-se.html">
            
                <a href="../../axon-server/installation/local-installation/axon-server-se.html">
            
                    
                    Axon Server SE
            
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        <li class="chapter " data-level="4.2.1.2" data-path="../../axon-server/installation/local-installation/axon-server-ee.html">
            
                <a href="../../axon-server/installation/local-installation/axon-server-ee.html">
            
                    
                    Axon Server EE
            
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        <li class="chapter " data-level="4.2.2" data-path="../../axon-server/installation/docker-k8s/">
            
                <a href="../../axon-server/installation/docker-k8s/">
            
                    
                    Docker / K8s
            
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        <li class="chapter " data-level="4.2.2.1" data-path="../../axon-server/installation/docker-k8s/axon-server-se.html">
            
                <a href="../../axon-server/installation/docker-k8s/axon-server-se.html">
            
                    
                    Axon Server SE
            
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        <li class="chapter " data-level="4.2.2.2" data-path="../../axon-server/installation/docker-k8s/axon-server-ee.html">
            
                <a href="../../axon-server/installation/docker-k8s/axon-server-ee.html">
            
                    
                    Axon Server EE
            
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        <li class="chapter " data-level="4.3" data-path="../../axon-server/administration/">
            
                <a href="../../axon-server/administration/">
            
                    
                    Administration
            
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        <li class="chapter " data-level="4.3.1" data-path="../../axon-server/administration/admin-configuration/">
            
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                    Configuration
            
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        <li class="chapter " data-level="4.3.1.1" data-path="../../axon-server/administration/admin-configuration/configuration.html">
            
                <a href="../../axon-server/administration/admin-configuration/configuration.html">
            
                    
                    System Properties
            
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        <li class="chapter " data-level="4.3.1.2" data-path="../../axon-server/administration/admin-configuration/command-line-interface.html">
            
                <a href="../../axon-server/administration/admin-configuration/command-line-interface.html">
            
                    
                    Command Line Interface
            
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        <li class="chapter " data-level="4.3.1.3" data-path="../../axon-server/administration/admin-configuration/rest-api.html">
            
                <a href="../../axon-server/administration/admin-configuration/rest-api.html">
            
                    
                    REST API
            
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        <li class="chapter " data-level="4.3.2" data-path="../../axon-server/administration/monitoring/">
            
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                    Monitoring
            
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        <li class="chapter " data-level="4.3.2.1" data-path="../../axon-server/administration/monitoring/actuator-endpoints.html">
            
                <a href="../../axon-server/administration/monitoring/actuator-endpoints.html">
            
                    
                    Actuator Endpoints
            
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        <li class="chapter " data-level="4.3.2.2" data-path="../../axon-server/administration/monitoring/grpc-metrics.html">
            
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                    gRPC Metrics
            
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        <li class="chapter " data-level="4.3.2.3" data-path="../../axon-server/administration/monitoring/heartbeat-monitoring.html">
            
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                    Heartbeat Monitoring
            
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                                <h1 id="command-dispatchers">Command Dispatchers</h1>
<p>The <a href="command-handlers.html">Command Handlers</a> pages provide the background on how to handle command messages in your application. The dispatching process is the starting point of such a command message. Axon provides two interfaces you can use to send the commands to your command handlers, being:</p>
<ol>
<li>The Command Bus, and</li>
<li>The Command Gateway</li>
</ol>
<p>This page will show how and when to use the command gateway and bus. How to configure and specifics on the command gateway and bus implementations are discussed in the <a href="infrastructure.html">infrastructure</a> section.</p>
<h2 id="the-command-bus">The Command Bus</h2>
<p>The &apos;Command Bus&apos; is the mechanism that dispatches commands to their respective command handlers. As such it is the infrastructure component that is aware which component can handle which command.</p>
<p>Each command is always sent to exactly one command handler. If no command handler is available for the dispatched command, a <code>NoHandlerForCommandException</code> exception is thrown.</p>
<p>The <code>CommandBus</code> provides two methods to dispatch commands to their respective handler, being the <code>dispatch(CommandMessage)</code> and <code>dispatch(CommandMessage, CommandCallback)</code> methods:</p>
<pre><code class="lang-java"><span class="hljs-keyword">private</span> CommandBus commandBus; <span class="hljs-comment">// 1.</span>

<span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">dispatchCommands</span><span class="hljs-params">()</span> </span>{
    String cardId = UUID.randomUUID().toString(); <span class="hljs-comment">// 2.</span>

    <span class="hljs-comment">// 3. &amp; 4.</span>
    commandBus.dispatch(GenericCommandMessage.asCommandMessage(<span class="hljs-keyword">new</span> IssueCardCommand(cardId, <span class="hljs-number">100</span>, <span class="hljs-string">&quot;shopId&quot;</span>)));

    <span class="hljs-comment">// 5. &amp; 6.</span>
    commandBus.dispatch(
            GenericCommandMessage.asCommandMessage(<span class="hljs-keyword">new</span> IssueCardCommand(cardId, <span class="hljs-number">100</span>, <span class="hljs-string">&quot;shopId&quot;</span>)),
            (CommandCallback&lt;IssueCardCommand, String&gt;) (cmdMsg, cmdResultMsg) -&gt; {
                <span class="hljs-comment">// 7.</span>
                <span class="hljs-keyword">if</span> (cmdResultMsg.isExceptional()) {
                    Throwable throwable = cmdResultMsg.exceptionResult();
                } <span class="hljs-keyword">else</span> {
                    String commandResult = cmdResultMsg.getPayload();
                }
            }
    );
}
<span class="hljs-comment">// omitted class, constructor and result usage</span>
</code></pre>
<p>The <code>CommandDispatcher</code> described above exemplifies a couple of important aspects and capabilities of the dispatching commands:</p>
<ol>
<li>The <code>CommandBus</code> interface providing the functionality to dispatch command messages.</li>
<li><p>The aggregate identifier is, per best practice, initialized as the String of a random unique identifier.</p>
<p>Typed identifier objects are also possible, as long as the object implements a sensible <code>toString()</code> function.</p>
</li>
<li><p>The <code>GenericCommandMessage#asCommandMessage(Object)</code> method is used to create a <code>CommandMessage</code>.</p>
<p>To be able to dispatch a command on the <code>CommandBus</code>, you are required to wrap your own command object (e.g. the &apos;command message payload&apos;) in a <code>CommandMessage</code>.</p>
<p>The <code>CommandMessage</code> also allows the addition of <a href="../messaging-concepts/anatomy-message.html#meta-data">MetaData</a> to the Command Message.</p>
</li>
<li><p>The <code>CommandBus#dispatch(CommandMessage)</code> function will dispatch the provided <code>CommandMessage</code> on the bus, for delivery to a command handler.</p>
<p>If an application isn&apos;t directly interested in the outcome of a command, this method can be used.</p>
</li>
<li><p>If the outcome of command handling is relevant for your application, the optional second parameter can be provided, the <code>CommandCallback</code>.</p>
<p>The <code>CommandCallback</code> allows the dispatching component to be notified when command handling is completed.</p>
</li>
<li><p>The Command Callback has one function, <code>onResult(CommandMessage, CommandResultMessage)</code>, which is called when command handling has finished.</p>
<p>The first parameter is the dispatched command, whilst the second is execution result of the dispatched command.</p>
<p>Lastly, the <code>CommandCallback</code> is a &apos;functional interface&apos; due to <code>onResult</code> being its only method.</p>
<p>As such, <code>commandBus.dispatch(commandMessage, (cmdMsg, commandResultMessage) -&gt; { /* ... */ })</code> would also be possible.</p>
</li>
<li><p>The <code>CommandResultMessage</code> provides the API to verify whether command execution was exceptional or successful.</p>
<p>If <code>CommandResultMessage#isExceptional</code> returns true, you can assume that the <code>CommandResultMessage#exceptionResult</code> will return a <code>Throwable</code> instance containing the actual exception.</p>
<p>Otherwise, the <code>CommandResultMessage#getPayload</code> method <em>may</em> provide you with an actual result or <code>null</code>, as further specified here.</p>
</li>
</ol>
<blockquote>
<p><strong>Command Callback consideration</strong></p>
<p>In the case that <code>dispatch(CommandMessage, CommandCallback)</code> is used, the calling component may <em>not</em> assume that the callback is invoked in the same thread that dispatched the command. If the calling thread depends on the result before continuing, you can use the <code>FutureCallback</code>. The <code>FutureCallback</code> is a combination of a <code>Future</code> (as defined in the java.concurrent package) and Axon&apos;s <code>CommandCallback</code>. Alternatively, consider using a <code>CommandGateway</code>.</p>
</blockquote>
<h2 id="the-command-gateway">The Command Gateway</h2>
<p>The &apos;Command Gateway&apos; is a convenience approach towards dispatching commands. It does so by abstracting certain aspects for you when dispatching a command on the <code>CommandBus</code>. It this uses the <code>CommandBus</code> underneath to perform the actual dispatching of the message.<br>While you are not required to use a gateway to dispatch commands, it is generally the easiest option to do so.</p>
<p>The <code>CommandGateway</code> interface can be separated in two sets of methods, namely <code>send</code> and <code>sendAndWait</code>:</p>
<pre><code class="lang-java"><span class="hljs-keyword">private</span> CommandGateway commandGateway; <span class="hljs-comment">// 1.</span>

<span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">sendCommand</span><span class="hljs-params">()</span> </span>{
    String cardId = UUID.randomUUID().toString(); <span class="hljs-comment">// 2.</span>

    <span class="hljs-comment">// 3.</span>
    CompletableFuture&lt;String&gt; futureResult = commandGateway.send(<span class="hljs-keyword">new</span> IssueCardCommand(cardId, <span class="hljs-number">100</span>, <span class="hljs-string">&quot;shopId&quot;</span>));
}
<span class="hljs-comment">// omitted class, constructor and result usage</span>
</code></pre>
<p>The <code>send</code> API as shown above introduces a couple of concepts, marked with numbered comments:</p>
<ol>
<li><p>The <code>CommandGateway</code> interface providing the functionality to dispatch command messages.</p>
<p>It does so by internally leveraging the <code>CommandBus</code> interface <a href="command-dispatchers.html#the-command-bus">dispatch messages</a>.</p>
</li>
<li><p>The aggregate identifier is, per best practice, initialized as the String of a random unique identifier.</p>
<p>Typed identifier objects are also possible, as long as the object implements a sensible <code>toString()</code> function.</p>
</li>
<li><p>The <code>send(Object)</code> function requires a single parameter, the command object.</p>
<p>This is an asynchronous approach to dispatching commands.</p>
<p>As such the response of the <code>send</code> method is a <code>CompletableFuture</code>.</p>
<p>This allows for chaining of follow up operations <em>after</em> the command <a href="command-dispatchers.html#command-dispatching-results">result</a> has been returned.</p>
</li>
</ol>
<blockquote>
<p><strong>Callback when using <code>send(Object)</code></strong></p>
<p>The <code>CommandGateway#send(Object)</code> method uses the <code>FutureCallback</code> under the hood to unblock the command dispatching thread from the command handling thread.</p>
</blockquote>
<p>A synchronous approach to sending messages can also be achieved, by using the <code>sendAndWait</code> methods:</p>
<pre><code class="lang-java"><span class="hljs-keyword">private</span> CommandGateway commandGateway;

<span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">sendCommandAndWaitOnResult</span><span class="hljs-params">()</span> </span>{
    IssueCardCommand commandPayload = <span class="hljs-keyword">new</span> IssueCardCommand(UUID.randomUUID().toString(), <span class="hljs-number">100</span>, <span class="hljs-string">&quot;shopId&quot;</span>);
    <span class="hljs-comment">// 1.</span>
    String result = commandGateway.sendAndWait(commandPayload);

    <span class="hljs-comment">// 2.</span>
    result = commandGateway.sendAndWait(commandPayload, <span class="hljs-number">1000</span>, TimeUnit.MILLISECONDS);
}
<span class="hljs-comment">// omitted class, constructor and result usage</span>
</code></pre>
<ol>
<li><p>The <code>CommandGateway#sendAndWait(Object)</code> function takes in a single parameter, your command object.</p>
<p>It will wait indefinitely until the command dispatching and handling process has been resolved.</p>
<p>The result returned by this method can either be successful or exceptional, as will be explained <a href="command-dispatchers.html#command-dispatching-results">here</a>.</p>
</li>
<li><p>If waiting indefinitely is not desirable, a &apos;timeout&apos; paired with the &apos;time unit&apos; can be provided along side the command object.</p>
<p>Doing so will ensure that the command dispatching thread will not wait longer than specified.</p>
<p>If command dispatching/handling was interrupted or the timeout was reached whilst using this approach, the command result will be <code>null</code>.</p>
<p>In all other scenarios, the result follows the <a href="command-dispatchers.html#command-dispatching-results">referenced</a> approach.</p>
</li>
</ol>
<h2 id="command-dispatching-results">Command Dispatching Results</h2>
<p>Dispatching commands will, generally speaking, have two possible outcomes:</p>
<ol>
<li>Command handled successfully, and</li>
<li>command handled exceptionally</li>
</ol>
<p>The outcome to some extent depends on the dispatching process, but more so on the implementation of the command handler. Thus if the <code>@CommandHandler</code> annotated function throws an exception due to some business logic, it will be that exception which will be the result of dispatching the command.</p>
<p>The successful resolution of command handling intentionally <em>should not</em> provide any return objects. Thus, if the <code>CommandBus</code>/<code>CommandGateway</code> provides a response (either directly or through the <code>CommandResultMessage)</code>, then you should assume the result of successful command handling to return <code>null</code>.</p>
<p>While it is possible to return results from command handlers, this should be used sparsely. The intent of the Command should never be to retrieve a value, as that would be an indication that the message should be designed as a <a href="../queries">Query Message</a>. Exceptions to this would be the identifier of the Aggregate Root, or identifiers of entities the Aggregate Root has instantiated. The framework has one such exception build in, on the <code>@CommandHandler</code> annotated constructor of an Aggregate. In case the &apos;command handling constructor&apos; has executed successfully, instead of the Aggregate itself, the value of the <code>@AggregateIdentifier</code> annotated field will be returned.</p>
<p><a href="https://youtu.be/lxonQnu1txQ" target="_blank">Axon Coding Tutorial #5: - Connecting the UI</a></p>

                                
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